Nonlinear responses of the majority of power amplifiers are not ideal due to influences of devices and an environment, thus resulting amplitude and phase distortion of in-band and out-of-band signals. Particularly a wideband multi-carrier wireless communication system being currently in active development and ready to be put into commercial use requires higher linearity and efficiency of a radio frequency power amplifier than a narrowband single-carrier system. The following problems of an application of a power amplifier result from the current development of 3G and 4G technologies.
1. Linearity of a wideband power amplifier is seriously deteriorated due to high transmission power, thus directly resulting in a deteriorated Error Vector Magnitude (EVM) of an in-band signal and exacerbated out-of-band interference, and a current test shows that interference power of an adjacent channel approximates to 0 dBm and an Adjacent Channel Leakage power Ratio (ACLR) of the adjacent channel and a next adjacent channel is about 35 dBc at rated transmission power of 43 dBm in the frequency band of 20.10 MHz to 2025 MHz, which is too far from an index of 45 dBc of a next adjacent channel stipulated by the 3rd Generation Partnership Project (3 GPP) to satisfy the requirement of the 3GPP.
2. A bandwidth of a signal constantly increases due to multi-carrier applications (9 carriers, 12 carriers and even more carriers), and also a peak to average power ratio of a transmission signal becomes very high due to applications of various complex modulation modes, e.g., 16-Quadrature Amplitude Modulation (16QAM), 64QAM, Orthogonal Frequency Division Multiplexing (OFDM), etc. A 12-carrier application of a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) system has a peak to average power ratio up to 16-17 dB which is limited to just roughly 9.5 dB even with phase rotation, and there is a peak to average power ratio of roughly 10 dB for Long Term Evolution-Time Division Duplex (LTE-TDD). If a linear characteristic of a power amplifier is ensured only with a power back-off, then the efficiency of the power amplifier will become very low, typically below 10%, and the absence of the power back-off will introduce so great nonlinear distortion that a receiver can not demodulate any signal.
3. A green base station requires an increase in the efficiency of a power amplifier from past 10% below to 30% above with the efficiency of a system also above 20%, and a study of existing envelop tracking and other technologies shows potential efficiency of a system above 40%.
The foregoing problems inspire a demand for a technology to increase the efficiency of a power amplifier, and there is an inherent contradiction between the linearity and efficiency of a power amplifier. If it is desired to address the foregoing problems only with an improvement of power amplifier technologies and processes, then existing devices might hardly have been satisfactory. Thus it is very curial to improve a linearity characteristic of a power amplifier while ensuring the efficiency thereof with the existing power amplifier technologies. The technology of Digital Pre-Distortion (DPD) is one of the most effective power amplifier linearization technologies to address the contradiction, and it compensates nonlinearity of a power amplifier effectively in the digital domain with existing powerful signal processing technologies to provide an output signal at greater power while reducing the volume, power consumption and cost of an apparatus.
FIG. 1 is a schematic diagram of a general digital pre-distortion solution, and a currently common DPD solution is as illustrated in FIG. 1. An input Inphase/Quadrature (I/Q) signal passes a Digital Up-Converter (DUC) and undergoes multi-carrier aggregation and then peak to average power ratio suppression (e.g. Crest Factor Reduction, CFR with a crest factor being equal to the square root of a peak to average power ratio), and then a transmission signal undergoes a pre-distortion process, passes a Digital-to-Analogue Converter (DAC) and undergoes carrier modulation and then enters a Power Amplifier (PA) generating through coupling an output signal of the power amplifier, which is transmitted back to the digital pre-distorting module over a feedback channel. Pre-distortion performance is greatly influenced by both a bit width and a sampling bandwidth of an Analogue-to-Digital Converter (ADC) of the feedback channel, and the digital pre-distorting module keeps making a statistic of a characteristic of the signal and will not start to train coefficients until an appropriate signal is found. A signal can be selected on such a criterion as a dynamic range, a peak to Average Power Ratio (PAPR), power, amplitude, a phase and other characteristic of the signal, possibly also in combination with a frame structural characteristic (e.g., including a special timeslot) or a specific transmission design.
In some cases, there is a regular characteristic of a signal in the time domain, and then parameters can be estimated periodically with a specific segment of the signal. At this time it is not necessary to make a statistic of the signal but instead acquisition of data is triggered with an external trigger signal, and for example, in a Time Division Duplex (TDD) frame structure of Worldwide interoperability over Microwave Access (WiMAX) based upon the technology of Orthogonal Frequency Division Multiplexing (OFDM), it is recognized that the part of a preamble is at high power and there is a significant signal range after CFR, so good options of samples for estimating parameters are possible. If such a characteristic in the time domain is absent with the signal, then samples are acquired randomly and a signal is decided, and no parameter will be estimated until qualified samples are acquired, thereby ensuring correction of all of estimated parameters.
Digital pre-distortion is performed in the prior art primarily based upon dynamical tracking of a change in a signal, and this method works well for a single-antenna application because a DPD coefficient training module is dedicated to the channel and can track in real time a change in the signal and thus compensate in a timely manner distortion resulting from nonlinearity of a power amplifier.
Unfortunately this method has the following at least two drawbacks: one drawback lies in that it has to operate in real time because it is unknown that whether the signal can be used to train coefficients and thus a change in the signal has to be tracked all the time and compared with a signal template resulting from a long-term statistic to trigger training, thus posing another constraint on essentially insufficient digital intermediate-frequency resources; and the other drawback lies in that each antenna has to be configured separately with a corresponding DPD feedback channel and coefficient training module so as to accommodate a multi-antenna application, and this will greatly increase the volume and cost of an apparatus along with the increasing number of antennas, thus resulting in poor feasibility from the perspective of commercialization.
Furthermore in the WiMAX system based upon the technology of OFDM, although there is high transmission power of a downlink preamble signal, boosted carrier power is supported in a data zone and thus the power of a real transmission signal of the preamble may not necessarily be the highest; and also the peak-to-average ratio of the preamble below 5 dB is too low to satisfy a requirement on a training signal for digital pre-distortion; and neither broadcast channel nor Downlink Pilot Time Slot (DwPTS) signal of TD-SCDMA accommodates a required characteristic of the training signal, so a specific signal has to be considered for DPD coefficient training.